The invention concerns carrier-current telecommunications and more particularly frequency translation circuits operating with inversion modulation.
Such a circuit, which is described in French Pat. No. 739,103 filed on June 25th, 1932, by Societe d'Etudes pour Liaisons Telephoniques et Telegraphiques, is illustrated in FIG. 1. Between a first pair of terminals 1 and 2 and a second pair of terminals 5 and 6, the said circuit comprises two transformers 9 and 10 which are connected by the diodes 11, 12, 13 and 14. The transformers 9 and 10 each have a winding whose mid-point is connected respectively to one of the terminals 3 and 4 of a third pair of terminals. Two pairs of terminals are respectively fed with a carrier wave at frequency F and a signal at frequency f. The waves translated to the frequencies F-f and F+f respectively, which result from the inversion modulation, are picked up between the pair of terminals which has remained free. The quality of such a circuit depends essentially upon the couplings between the various pairs of terminals and the non-linear distortions which build up undesirable frequencies in the circuit. Reduction of the couplings involves balancing of the windings of the transformers as well as matching of the diodes. With regard to the non-linear distortion, it is a function both of the characteristics of the diodes and of the wave shape and the power level of the carrier wave. In practice, in carrier current systems utilizing this method of frequency transposition, the carrier wave power necessary to each channel is of the order of 300 microwatts and may reach 10 milliwatts when the number of channels to be translated rises to 900. Generally speaking, the oscillator at the frequency F of the carrier wave, which is common to a number of translation circuits, supplies insufficient power, so that an amplifier which raises the power of the carrier wave to the necessary value must be associated with each frequency translation circuit. The increasing capacity of analog multiplex equipments results in an increase in the number of translation circuits and of associated amplifiers and consequently leads to the use of a considerable supply power. Since the energy output of the circuits is low, an unacceptable temperature rise takes place in the equipment.
The present invention has for its object to provide a frequency translation circuit which requires a low carrier wave level, which results in a limitation of the necessary total power and consequently of the power dissipated in the form of heat by the frequency translation circuits in a communication system comprising a large number of channels.